The function of a nerve terminal is to release transmitter molecules at the right time and the right place. Transmitter is stored inside synaptic vesicles in nerve terminals until vesicles fuse with the plasma membrane, releasing their contents. After release, new synaptic vesicles re-appear within seconds. Using cell-free reconstitution assays, the applicants have discovered that there are two pathways of synaptic vesicle formation. Naturally occurring mutants in mice lack one of these pathways, the one that utilizes the adaptor complex AP3. Such mice have major behavioral abnormalities, resembling aberrant behavior found in humans. This proposal aims at understanding the basic molecular mechanisms in the AP3 pathway, in order to understand the behavior and lay the basis for developing novel drug therapies. It also compares the two pathways of vesicle biogenesis, looking for differences in molecular mechanism, physiological relevance and development sequence.